1. Field of the Invention
This invention pertains to the recovery of heavy oils and tars from steeply dipping reservoirs penetrated by a plurality of wells and more particularly to steam flooding operations involving same.
2. Description of the Prior Art
Petroleum reservoirs are found in an almost incredible variety. Those of interest herein are steeply dipping reservoirs which outcrop at the surface and contain predominately high viscosity petroleums fractions such as heavy oils and tars.
In a steeply dipping reservoir the most commonly used production technique is gravity drainage wherein production wells are drilled to the bottommost portions of the reservoir allowing the oil to flow downdip under the influence of gravity to the production wells wherein the petroleum is either flowed or pumped to the surface. The rate of the downward oil flow is known to be proportional to a term: EQU K.sub.o /.mu..sub.o (.rho..sub.o -.rho..sub.g) sin .alpha.
where K.sub.o is the oil permeability, .mu..sub.o is the oil viscosity, .rho..sub.o is oil density, .rho..sub.g is gas density, and .alpha. is the reservoir dip angle. For reservoirs exhibiting strong gravity drainage characteristics, the value of the above term ranges from 10-200 when K.sub.o is expressed in millidarcies, .mu..sub.o in centipoise, .rho..sub.o and .rho..sub.g in grams per cubic centimeter. It is immediately evident that for a steeply dipping reservoir containing highly viscous petroleum, the value of the above term at the initial reservoir temperature will be much less than 10 due to the high value of the oil viscosity. Basic reservoir engineering knowledge indicates that the most effective means to reduce the oil viscosity in such situations is to inject steam or hot water into the reservoir. The heat from the injected fluids serves to raise the temperature of the reservoir with a resulting reduction on the viscosity of the petroleum contained therein. Consequently, the value of the above term can be increased to within the desirable range of 10-200, thereby creating a favorable gravity drainage condition for a steeply dipping reservoir containing high viscosity petroleum.
Nevertheless, the injection of steam into the updip portion of an outcropping reservoir presents a number of problems. The optimum sequence of events for such a steam injection program would comprise first an initial mobilization of the petroleum in the vicinity of the steam injection wells formed by the formation of a bank of the mobilized oil followed by the displacement of the bank downwards toward the production wells by the continued injection of steam into the updip injection wells. Unfortunately, the natural tendency of the steam, due to its low density and high mobility, is to flow upwards in the formation to the updip limit of the reservoir at the outcrop. Indeed, uncontrolled steam injection can easily result in a condition wherein the steam front breaks through at the outcrop. Such a steam breakthrough would severely damage the reservoir's potential for further recovery of petroleum as well as create serious environmental pollution problems.
Heretofor, avoidance of steam breakthrough at the outcrop has been achieved only by those methods which employ extremely conservative steam injection rates and the shutting in of any potential steam injection wells which were felt to be in too close proximity to the outcrop. Such production practices, while prudent, will often leave substantial areas of the reservoir essentially untapped by the steam injection program due to the low injection rates and avoidance of the upper portions of the reservoir in the fear of a steam breakthrough. There remains an unmet need to utilize the full potential of an efficient steam injection program in a steeply dipping heavy oil reservoir while concurrently avoiding the problem of steam breakthrough at the outcrop.